Experimental Study of Silane Plasma Nanoparticle Formation in Amorphous Silicon Thin Films
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Experimental Study of Silane Plasma Nanoparticle Formation in Amorphous Silicon Thin Films S. Thompson1, C. R. Perrey2, T. J. Belich3, C. Blackwell3, C. B. Carter2, J. Kakalios3 and U. Kortshagen1 Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 3 School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 1 2
ABSTRACT RF glow discharge deposited hydrogenated amorphous silicon films containing silicon nanocrystalline inclusions (a/nc-Si:H) films are investigated as a function of a thermal gradient applied across the silane plasma during film growth. The a/nc-Si:H films are synthesized from hydrogen-diluted silane plasmas when a capacitively-coupled plasma enhanced chemical vapor deposition reactor is operated at high gas chamber pressures. Plasma diagnostics and transmission electron microscopy image analysis of films deposited with and without a thermal gradient suggest that nanoparticle formation occurs within the plasma, rather than resulting from solid-state nucleation at the growing film surface. INTRODUCTION Hydrogenated amorphous silicon containing silicon nanocrystallites (a/nc-Si:H) have recently attracted considerable attention,1,2,3 motivated by reports that these materials exhibit a greater resistance to light-induced defect formation (the Staebler-Wronski effect (SWE)4) compared to hydrogenated amorphous silicon (a-Si:H).1-3,5 The modification of the strained-bond distribution of the amorphous network surrounding each silicon nanocrystallite may inhibit the light-induced degradation effect in a/nc-Si:H films.6 This material is typically synthesized in a plasma enhanced chemical vapor deposition (PECVD) system under plasma conditions (heavy hydrogen dilution of silane and chamber pressures significantly above 100 mTorr) near the edge of powder formation, where the powder consists of particles that may be several tens of nanometers in size within the plasma. The hydrogen dilution of the silane inhibits the growth of larger particles,7 and under certain conditions smaller silicon clusters and crystallites, with diameters as small as 1 nm, are formed in the plasma and incorporated into the growing film (as can be seen in the high resolution transmission electron microscope (HRTEM) image in Fig. 1).8 Fig. 1: HRTEM image of silicon nanocrystal in a/ncSi:H film deposited with no thermal gradient, R = 50 and gas chamber pressure of 1450 mTorr.
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The particulates and clusters formed within the plasma can be directed either towards or away from the growing film surface by a thermophoretic force induced by a thermal gradient across the electrodes in the plasma.9 The traditional deposition configuration in a PECVD reactor, with the grounded electrode heated and the RF electrode unheated would orient the thermophoretic force so as to push any particulates in the silane plasma away from the growing film surface. Here we report measurements of
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